Four decades of post-Chernobyl 137Cs vertical migration in undisturbed soil in Northern Greece: a parsimonious...

This study investigates the long-term vertical migration of 137Cs in undisturbed grassland soil using a unique 40-year monitoring dataset (1987-2026) from Northern Greece established after the Chernobyl accident. Fractional 137Cs depth distributions measured in six 5 cm soil layers (0-30 cm) were analysed using a parsimonious 1D process-based model incorporating advection, hydrodynamic dispersion, and an effective irreversible transfer from a migrating to a fixed phase. Model parameters were estimated through hybrid global-local optimization, and their uncertainty was evaluated by residual bootstrap analysis. A single parameter set successfully reproduced the temporal evolution of all measured depth profiles, yielding strong overall agreement with observations. The estimated effective transport parameters were an advective migration velocity of 0.217 ± 0.093 cm y-1, a dispersion coefficient of 0.24 ± 0.48 cm2 y-1, and an effective fixation coefficient of 0.0365 ± 0.0178 y-1. The dispersion coefficient exhibited substantial uncertainty because of the limited vertical resolution (5 cm) sampling intervals, whereas the fixation parameter was more robustly constrained and represents the residual long-term transfer required to reproduce the observed profile evolution after the initial rapid postdepositional fixation period. These results demonstrate that a parsimonious process-based model provides an effective description of the long-term redistribution of 137Cs at the monitored undisturbed grassland site in Northern Greece.